The fundamental construction of screw machines designed for example as screw compressors is known to the person skilled in the art addressed here. Accordingly, a screw compressor comprises a machine housing or a compressor housing, which comprises a first housing section and a second housing section. In the first housing section, screw rotors are mounted which form a rotor pair and serve for compressing a medium to be compressed. Likewise, a control slide valve is typically mounted in the first housing section, which in sections delimits a working space or a compression space of the first housing section and for changing the size of the effective working space or effective compression space can be shifted parallel to the rotor axes of the screw rotors.
From DE 30 22 202 C2 and from DE 38 03 044 A1, the content of which is incorporated herein in their entirety, the fundamental construction of screw machines designed for example as screw compressors is known. Accordingly, a screw compressor comprises a machine housing or a compressor housing, wherein in the machine housing or compressor housing screw rotors are mounted which form a rotor pair. The screw rotors of the screw compressor in this case are mounted in the machine housing via axial bearings and radial bearings. Seals assume the sealing of the screw rotors, in particular the sealing of shaft passages of the screw rotors in the machine housing.
It is known, furthermore, to supply the bearings and seals of the screw rotors of such a screw machine with oil starting out from an oil supply system in order to lubricate and/or cool the bearings and seals of the screw rotors. To this end, the oil supply system of the screw machine comprises an oil feed in order to feed the oil to the bearings and seals to be lubricated and/or cooled and comprises an oil discharge or oil return in order to subsequently discharge the oil from the bearings and seals.
In the case of so-called oil-injected screw compressors, the oil, which is discharged from the bearings and seals via the oil discharge or oil return is conducted in the direction of the compression space of the screw compressor starting out from the oil discharge via an oil feed, which is also called feed location here, wherein the oil in the compression space mixes with the working medium, in particular with a gas to be compressed and is then pushed out from the screw machine.
The oil feed or the feed location, via which the oil discharged from the bearings and seals can be conducted in the direction of the compression space of the screw compressor, is located, in many screw compressors, seen in delivery direction behind a termination of the compression chamber towards the suction side of the screw compressor. In the oil discharge or oil return or the oil feed, a pressure is present the knowledge of which is necessary in order to supply the bearings and seals of the screw machine with an adequate quantity of oil for cooling and/or lubricating. Accordingly, it is provided with screw machines known from practice to determine the pressure in the oil discharge or oil return or the pressure in the oil feed branching off the oil discharge or oil return through calculation, in order to dependent on this build up an adequately high pressure difference on the bearings and/or seals to be cooled and/or lubricated in order to ultimately supply the bearings and seals with an adequate quantity of oil for cooling and/or lubricating.
The determination through calculation of the pressure in the region of the oil discharge or oil return or oil feed is inaccurate and can greatly fluctuate with changing suction pressure of the screw machine. Furthermore, this pressure is dependent on a so-called isentropic exponent of the working medium, in particular of a gas to be compressed. The position of a control slide valve of the screw machine that may be present can also affect this pressure. Since the determination by calculation of the pressure in the oil discharge or oil return or oil feed accordingly is inaccurate and can be subject to great fluctuations, the pressure determined by calculation is always offset by a high safety allowance in the case of screw machines known from practice, in order to always ensure a secure oil supply of the bearings and seals. Here, the appropriate pressure for the bearings and for the seals in screw machines known from practice is adjusted in each case via separate pressure controllers.
The disadvantages of such screw machines known from practice consists in that too much oil is conducted in the direction of the bearings and seals because of the determination of the pressure in the oil discharge or oil return or oil feed which is only relatively inaccurate and of the relatively high safety allowance or offset. In the case that the suction pressure of the screw machine rises unexpectedly high, a secure oil supply of the bearings and seals can no longer be ensured when a minimum pressure difference is undershot. Because of this, the bearings and seals can be damaged and impaired in their function. There is therefore a need for a screw machine and a method for operating the same, with the help of which the disadvantages known from practice can be avoided. Starting out from this, the invention is based on the object of creating a new type of screw machine and a method for operating the same.